This invention relates to a varistor, and more particularly to a cylindrical varistor whose properties are dependent upon the geometry of a gap separating the electrodes and a method of making the same. A varistor is a voltage variable resistor, and its electrical behavior is commonly described by the following characteristic relationship: EQU I = (V/C).sup..alpha.
wherein:
I = current flowing through the varistor; PA1 V = voltage across the varistor; PA1 C = constant; and PA1 .alpha. = constant &gt; 1; measure of the non-linearity of the varistor.
A number of varistors are known in the art which can be referred to as bulk type varistors. A bulk device is disclosed in U.S. Pat. No. 3,496,512, issued on Feb. 17, 1970 to Matsuoka, et al. for "Non-Linear Resistors," having a sintered body of zinc oxide with silver paint electrodes applied to opposite surfaces. The properties are dependent upon the bulk of the device; i.e., the non-linearity is determined to a considerable extent by the composition of the sintered body, and the value of C is controlled by the dimension of the body between the electrodes. There are many other devices of a similar construction in which the sintered body includes together with zinc oxide various metal oxides to effectuate an increase in the non-linearity property. See, for example, U.S. Pat. No. 3,632,528, issued on Jan. 4, 1972 to Matsuoka, et al. for "Lead-Modified Zinc Oxide Voltage Variable Resistor;" U.S. Pat. No. 3,634,337, issued on Jan. 11, 1972 to Matsuoka, et al. for "Barium-Modified Zinc Oxide Voltage Variable Resistor;" U.S. Pat. No. 3,598,763, issued on Aug. 10, 1971 to Matsuoka, et al. for "Manganese-Modified Zinc Oxide Voltage Variable Resistor;" and U.S. Pat. No. 3,699,058 issued on Oct. 17, 1972 to Matsuoka, et al. for "Uranium-Modified Zinc Oxide Voltage Variable Resistor."
Such varistors have been fabricated in both disc and cylindrical shapes--a varistor material is initially formed into the desired shape and an electrode is applied to each end. A lead wire is then attached to each electrode, and this step is followed by enclosure of the varistor material and the electrodes within a conformal coating. Although the properties of the varistors can be varied by adjusting the thickness of the varistor material, there is difficulty in forming the material to achieve precise results.
It is also important that a strong bond between the leads and the varistor body is obtained. A significant problem has arisen in the bulk devices presently available in that there has been a tendency for the electrodes to splinter in the vicinity of the attachments of the leads. Also, the electrodes in some devices have become dissociated from the varistor material. Either occurrence can result in a failure of a varistor device thus introducing transient voltages into a circuit that is to be protected. In order to overcome these difficulties, the patent issued to May, U.S. Pat. No. 3,903,404, on Sept. 2, 1975 for "Metal Oxide Varistor With Coating That Enhances Contact Adhesion" provides a coating which is applied to the varistor material before the electrodes are applied. Although this approach can result in an improved bond between the electrodes and the varistor body, the attachment between the electrodes and the lead wires does not appear to be appreciably strengthened. Also, the electrodes still may lose some degree of structural integrity near the leads due to mechanical stresses.
Recently, other varistors have been developed in which the properties are independent of the dimensions of the varistor body. In U.S. Pat. No. 3,768,058, issued on Oct. 23, 1973 to Harnden, Jr. for "Metal Oxide Varistor With Laterally Spaced Electrodes," a pair of electrodes are applied to the same surface of the varistor body. Since the separation between the electrodes is less than the thickness of the body, the width of the separation determines the voltage level across the electrodes. This construction thus allows improved control over the voltage characteristics of the varistor for it is unnecessary to control the thickness of the varistor body. However, there is no disclosure of the use of this type of device in a cylindrical form; nor is there any indication that the device can be adapted to overcome the mechanical deficiencies observed in the bulk devices. The prior art also lacks and teaching of how the electrode separation can be controlled to adjust the voltage characteristics, as well as other varistor properties. Also, there is still a need for a method which can be employed to produce a varistor whose properties are independent of the size of the varistor body.
It is against this background that the present invention introduces a cylindrical varistor whose properties are readily adjusted during fabrication and which has mechanical advantages over presently existing bulk devices. A method of making such cylindrical varistors is also provided.